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<H1><a name="Go"></a>22 SWIG and Go</H1>
<!-- INDEX -->
<div class="sectiontoc">
<ul>
<li><a href="#Go_overview">Overview</a>
<li><a href="#Go_running_swig">Running SWIG with Go</a>
<ul>
<li><a href="#Go_commandline">Additional Commandline Options</a>
<li><a href="#Go_outputs">Go Output Files</a>
</ul>
<li><a href="#Go_basic_tour">A tour of basic C/C++ wrapping</a>
<ul>
<li><a href="#Go_package">Go Package Name</a>
<li><a href="#Go_names">Go Names</a>
<li><a href="#Go_constants">Go Constants</a>
<li><a href="#Go_enumerations">Go Enumerations</a>
<li><a href="#Go_classes">Go Classes</a>
<ul>
<li><a href="#Go_class_inheritance">Go Class Inheritance</a>
</ul>
<li><a href="#Go_templates">Go Templates</a>
<li><a href="#Go_director_classes">Go Director Classes</a>
<li><a href="#Go_primitive_type_mappings">Default Go primitive type mappings</a>
<li><a href="#Go_output_arguments">Output arguments</a>
<li><a href="#Go_adding_additional_code">Adding additional go code</a>
</ul>
</ul>
</div>
<!-- INDEX -->



<p>
This chapter describes SWIG's support of Go.  For more information on
the Go programming language
see <a href="http://golang.org/">golang.org</a>.
</p>

<H2><a name="Go_overview"></a>22.1 Overview</H2>


<p>
Go is a compiled language, not a scripting language.  However, it does
not support direct calling of functions written in C/C++.  The cgo
program may be used to generate wrappers to call C code from Go, but
there is no convenient way to call C++ code.  SWIG fills this gap.
</p>

<p>
There are (at least) two different Go compilers.  One is the gc
compiler, normally invoked under the names 6g, 8g, or 5g.  The other
is the gccgo compiler, which is a frontend to the gcc compiler suite.
The interface to C/C++ code is completely different for the two Go
compilers.  SWIG supports both, selected by a command line option.
</p>

<p>
Because Go is a type-safe compiled language, SWIG's runtime type
checking and runtime library are not used with Go.  This should be
borne in mind when reading the rest of the SWIG documentation.
</p>

<H2><a name="Go_running_swig"></a>22.2 Running SWIG with Go</H2>


<p>
To generate Go code, use the <tt>-go</tt> option with SWIG.  By
default SWIG will generate code for the gc compilers.  To generate
code for gccgo, you should also use the <tt>-gccgo</tt> option.
</p>

<H3><a name="Go_commandline"></a>22.2.1 Additional Commandline Options</H3>


<p>
These are the command line options for SWIG's GO module.  They can
also be seen by using:
</p>

<div class="code"><pre>
swig -go -help
</pre></div>

<table summary="Go specific options">
<tr>
<th>Go specific options</th>
</tr>

<tr>
<td>-gccgo</td>
<td>Generate code for gccgo.  The default is to generate code for
  6g/8g/5g.</td>
</tr>

<tr>
<td>-package &lt;name&gt;</td>
<td>Set the name of the Go package to &lt;name&gt;.  The default
  package name is the SWIG module name.</td>
</tr>

<tr>
<td>-soname %lt;name%gt;</td>
<td>Set the runtime name of the shared library that the dynamic linker
  should include at runtime.  The default is the package name with
  ".so" appended.  This is only used when generating code for
  6g/8g/5g; when using gccgo, the equivalent name will be taken from
  the <code>-soname</code> option passed to the linker.</td>
</tr>

<tr>
<td>-go-prefix &lt;prefix&gt;</td>
<td>When generating code for gccgo, set the prefix to use.  This
  corresponds to the <tt>-fgo-prefix</tt> option to gccgo.</td>
</tr>

<tr>
<td>-long-type-size &lt;s&gt;</td>
<td>Set the size for the C/C++ type <tt>long</tt>.  This controls
  whether <tt>long</tt> is converted to the Go type <tt>int32</tt>
  or <tt>int64</tt>.  The &lt;s&gt; argument should be 32 or 64.</td>
</tr>

</table>

<H3><a name="Go_outputs"></a>22.2.2 Go Output Files</H3>


<p> When generating Go code, SWIG will generate the following
  files:</p>

<ul>
<li>
MODULE.go will contain the Go functions that your Go code will call.
These functions will be wrappers for the C++ functions defined by your
module.  This file should, of course, be compiled with the Go
compiler.
<li>
MODULE_wrap.c or MODULE_wrap.cxx will contain C/C++ functions will be
invoked by the Go wrapper code.  This file should be compiled with the
usual C or C++ compiler and linked into a shared library.
<li>
MODULE_wrap.h will be generated if you use the directors feature.  It
provides a definition of the generated C++ director classes.  It is
generally not necessary to use this file, but in some special cases it
may be helpful to include it in your code, compiled with the usual C
or C++ compiler.
<li>
If using the gc compiler, MODULE_gc.c will contain C code which should
be compiled with the C compiler distributed as part of the gc compiler: 6c, 8c,
or 5c.  It should then be combined with the compiled MODULE.go using
gopack.  This file will not be generated when using gccgo.
</ul>

<p>
A typical command sequence would look like this:
</p>

<div class="code"><pre>
% swig -go example.i
% gcc -c -fpic example.c
% gcc -c -fpic example_wrap.c
% gcc -shared example.o example_wrap.o -o example.so
% 6g example.go
% 6c example_gc.c
% gopack grc example.a example.6 example_gc.6
% 6g main.go  # your code, not generated by SWIG
% 6l main.6
</pre></div>

<H2><a name="Go_basic_tour"></a>22.3 A tour of basic C/C++ wrapping</H2>


<p>
By default, SWIG attempts to build a natural Go interface to your
C/C++ code.  However, the languages are somewhat different, so some
modifications have to occur.  This section briefly covers the
essential aspects of this wrapping.
</p>

<H3><a name="Go_package"></a>22.3.1 Go Package Name</H3>


<p>
All Go source code lives in a package.  The name of this package will
default to the name of the module from SWIG's <tt>%module</tt>
directive.  You may override this by using SWIG's <tt>-package</tt>
command line option.
</p>

<H3><a name="Go_names"></a>22.3.2 Go Names</H3>


<p>
In Go, a function is only visible outside the current package if the
first letter of the name is uppercase.  This is quite different from
C/C++.  Because of this, C/C++ names are modified when generating the
Go interface: the first letter is forced to be uppercase if it is not
already.  This affects the names of functions, methods, variables,
constants, enums, and classes.
</p>

<p>
C/C++ variables are wrapped with setter and getter functions in Go.
First the first letter of the variable name will be forced to
uppercase, and then <tt>Get</tt> or <tt>Set</tt> will be prepended.
For example, if the C/C++ variable is called <tt>var</tt>, then SWIG
will define the functions <tt>GetVar</tt> and <tt>SetVar</tt>.  If a
variable is declared as <tt>const</tt>, or if
SWIG's <a href="SWIG.html#SWIG_readonly_variables">
<tt>%immutable</tt> directive</a> is used for the variable, then only
the getter will be defined.
</p>

<p>
C++ classes will be discussed further below.  Here we'll note that the
first letter of the class name will be forced to uppercase to give the
name of a type in Go.  A constructor will be named <tt>New</tt>
followed by that name, and the destructor will be
named <tt>Delete</tt> followed by that name.
</p>

<H3><a name="Go_constants"></a>22.3.3 Go Constants</H3>


<p>
C/C++ constants created via <tt>#define</tt> or the <tt>%constant</tt>
directive become Go constants, declared with a <tt>const</tt>
declaration.

<H3><a name="Go_enumerations"></a>22.3.4 Go Enumerations</H3>


<p>
C/C++ enumeration types will cause SWIG to define an integer type with
the name of the enumeration (with first letter forced to uppercase as
usual).  The values of the enumeration will become variables in Go;
code should avoid modifying those variables.
</p>

<H3><a name="Go_classes"></a>22.3.5 Go Classes</H3>


<p>
Go has interfaces, methods and inheritance, but it does not have
classes in the same sense as C++.  This sections describes how SWIG
represents C++ classes represented in Go.
</p>

<p>
For a C++ class <tt>ClassName</tt>, SWIG will define two types in Go:
an underlying type, which will just hold a pointer to the C++ type,
and an interface type.  The interface type will be
named <tt>ClassName</tt>.  SWIG will define a
function <tt>NewClassName</tt> which will take any constructor
arguments and return a value of the interface
type <tt>ClassName</tt>.  SWIG will also define a
destructor <tt>DeleteClassName</tt>.
</p>

<p>
SWIG will represent any methods of the C++ class as methods on the
underlying type, and also as methods of the interface type.  Thus C++
methods may be invoked directly using the
usual <tt>val.MethodName</tt> syntax.  Public members of the C++ class
will be given getter and setter functions defined as methods of the
class.
</p>

<p>
SWIG will represent static methods of C++ classes as ordinary Go
functions.  SWIG will use names like <tt>ClassNameMethodName</tt>.
SWIG will give static members getter and setter functions with names
like <tt>GetClassName_VarName</tt>.
</p>

<p>
Given a value of the interface type, Go code can retrieve the pointer
to the C++ type by calling the <tt>Swigcptr</tt> method.  This will
return a value of type <tt>SwigcptrClassName</tt>, which is just a
name for <tt>uintptr</tt>.  A Go type conversion can be used to
convert this value to a different C++ type, but note that this
conversion will not be type checked and is essentially equivalent
to <tt>reinterpret_cast</tt>.  This should only be used for very
special cases, such as where C++ would use a <tt>dynamic_cast</tt>.
</p>

<p>Note that C++ pointers to compound objects are represented in go as objects
themselves, not as go pointers.  So, for example, if you wrap the following
function:</p>
<div class="code">
<pre>
class MyClass {
  int MyMethod();
  static MyClass *MyFactoryFunction();
};

</pre>
</div>
<p>You will get go code that looks like this:</p>
<div class="code">
<pre>
type MyClass interface {
  Swigcptr() uintptr
  SwigIsMyClass()
  MyMethod() int
}

MyClassMyFactoryFunction() MyClass {
  // swig magic here
}
</pre>
</div>
<p>Note that the factory function does not return a go pointer; it actually
returns a go interface.  If the returned pointer can be null, you can check
for this by calling the Swigcptr() method.
</p>

<H4><a name="Go_class_inheritance"></a>22.3.5.1 Go Class Inheritance</H4>


<p>
C++ class inheritance is automatically represented in Go due to its
use of interfaces.  The interface for a child class will be a superset
of the interface of its parent class.  Thus a value of the child class
type in Go may be passed to a function which expects the parent class.
Doing the reverse will require an explicit type assertion, which will
be checked dynamically.
</p>

<H3><a name="Go_templates"></a>22.3.6 Go Templates</H3>


<p>
In order to use C++ templates in Go, you must tell SWIG to create
wrappers for a particular template instantation.  To do this, use
the <tt>%template</tt> directive.

<H3><a name="Go_director_classes"></a>22.3.7 Go Director Classes</H3>


<p>
SWIG's director feature permits a Go type to act as the subclass of a
C++ class with virtual methods.  This is complicated by the fact that
C++ and Go define inheritance differently.  In Go, structs can inherit
methods via anonymous field embedding.  However, when a method is
called for an embedded struct, if that method calls any other methods,
they are called for the embedded struct, not for the original type.
Therefore, SWIG must use Go interfaces to represent C++ inheritance.
</p>

<p>
In order to use the director feature in Go, you must define a type in
your Go code.  You must then add methods for the type.  Define a
method in Go for each C++ virtual function that you want to override.
You must then create a value of your new type, and pass a pointer to
it to the function <tt>NewDirectorClassName</tt>,
where <tt>ClassName</tt> is the name of the C++ class.  That will
return a value of type <tt>ClassName</tt>.
</p>

<p>
For example:
</p>

<div class="code">
<pre>
type GoClass struct { }
func (p *GoClass) VirtualFunction() { }
func MakeClass() ClassName {
	return NewDirectorClassName(&amp;GoClass{})
}
</pre>
</div>

<p>
Any call in C++ code to the virtual function will wind up calling the
method defined in Go.  The Go code may of course call other methods on
itself, and those methods may be defined either in Go or in C++.
</p>

<H3><a name="Go_primitive_type_mappings"></a>22.3.8 Default Go primitive type mappings</H3>


<p>
The following table lists the default type mapping from C/C++ to Go.
This table will tell you which Go type to expect for a function which
uses a given C/C++ type.
</p>

<table BORDER summary="Go primitive type mappings">
<tr>
<td><b>C/C++ type</b></td>
<td><b>Go type</b></td>
</tr>

<tr>
<td>bool</td>
<td>bool</td>
</tr>

<tr>
<td>char</td>
<td>byte</td>
</tr>

<tr>
<td>signed char</td>
<td>int8</td>
</tr>

<tr>
<td>unsigned char</td>
<td>byte</td>
</tr>

<tr>
<td>short</td>
<td>int16</td>
</tr>

<tr>
<td>unsigned short</td>
<td>uint16</td>
</tr>

<tr>
<td>int</td>
<td>int</td>
</tr>

<tr>
<td>unsigned int</td>
<td>uint</td>
</tr>

<tr>
<td>long</td>
<td>int32 or int64, depending on <tt>-long-type-size</tt></td>
</tr>

<tr>
<td>unsigned long</td>
<td>uint32 or uint64, depending on <tt>-long-type-size</tt></td>
</tr>

<tr>
<td>long long</td>
<td>int64</td>
</tr>

<tr>
<td>unsigned long long</td>
<td>uint64</td>
</tr>

<tr>
<td>float</td>
<td>float32</td>
</tr>

<tr>
<td>double</td>
<td>float64</td>
</tr>

<tr>
<td>char *<br>char []</td>
<td>string</td>
</tr>

</table>

<p>
Note that SWIG wraps the C <tt>char</tt> type as a character. Pointers
and arrays of this type are wrapped as strings.  The <tt>signed
char</tt> type can be used if you want to treat <tt>char</tt> as a
signed number rather than a character.  Also note that all const
references to primitive types are treated as if they are passed by
value.
</p>

<p>
These type mappings are defined by the "gotype" typemap.  You may change
that typemap, or add new values, to control how C/C++ types are mapped
into Go types.
</p>

<H3><a name="Go_output_arguments"></a>22.3.9 Output arguments</H3>


<p>Because of limitations in the way output arguments are processed in swig,
a function with output arguments will not have multiple return values.
Instead, you must pass a pointer into the C++ function to tell it where to
store the output value.  In go, you supply a slice in the place of the output
argument.</p>

<p>For example, suppose you were trying to wrap the modf() function in the
C math library which splits x into integral and fractional parts (and
returns the integer part in one of its parameters):</p>
<div class="code">
<pre>
double modf(double x, double *ip);
</pre>
</div>
<p>You could wrap it with SWIG as follows:</p>
<div class="code">
<pre>
%include &lt;typemaps.i&gt;
double modf(double x, double *OUTPUT);
</pre>
</div>
<p>or you can use the <code>%apply</code> directive:</p>
<div class="code">
<pre>
%include &lt;typemaps.i&gt;
%apply double *OUTPUT { double *ip };
double modf(double x, double *ip);
</pre>
</div>
<p>In Go you would use it like this:</p>
<div class="code">
<pre>
ptr := []float64{0.0}
fraction := modulename.Modf(5.0, ptr)
</pre>
</div>
<p>Since this is ugly, you may want to wrap the swig-generated API with
some <a href="#Embedded_go_code">additional functions written in go</a> that
hide the ugly details.</p>

<p>There are no <code>char&nbsp;*OUTPUT</code> typemaps.  However you can
apply the <code>signed&nbsp;char&nbsp;*</code> typemaps instead:</p>
<div class="code">
<pre>
%include &lt;typemaps.i&gt;
%apply signed char *OUTPUT {char *output};
void f(char *output);
</pre>
</div>

<H3><a name="Go_adding_additional_code"></a>22.3.10 Adding additional go code</H3>


<p>Often the APIs generated by swig are not very natural in go, especially if
there are output arguments.  You can
insert additional go wrapping code to add new APIs
with <code>%insert(go_wrapper)</code>, like this:</p>
<div class="code">
<pre>
%include &lt;typemaps.i&gt;
// Change name of what swig generates to Wrapped_modf.  This function will
// have the following signature in go:
//   func Wrapped_modf(float64, []float64) float64
%rename(wrapped_modf) modf(double x, double *ip);

%apply double *OUTPUT { double *ip };
double modf(double x, double *ip);

%insert(go_wrapper) %{

// The improved go interface to this function, which has two return values,
// in the more natural go idiom:
func Modf(x float64) (fracPart float64, intPart float64) {
  ip := []float64{0.0}
  fracPart = Wrapped_modf(x, ip)
  intPart = ip[0]
  return
}

%}
</pre>
</div>

<p>For classes, since swig generates an interface, you can add additional
methods by defining another interface that includes the swig-generated
interface.  For example,</p>
<div class="code">
<pre>
%rename(Wrapped_MyClass) MyClass;
%rename(Wrapped_GetAValue) MyClass::GetAValue(int *x);
%apply int *OUTPUT { int *x };

class MyClass {
 public:
  MyClass();
  int AFineMethod(const char *arg); // Swig's wrapping is fine for this one.
  bool GetAValue(int *x);
};

%insert(go_wrapper) %{

type MyClass interface {
  Wrapped_MyClass
  GetAValue() (int, bool)
}

func (arg SwigcptrWrapped_MyClass) GetAValue() (int, bool) {
  ip := []int{0}
  ok := arg.Wrapped_GetAValue(ip)
  return ip[0], ok
}

%}
</pre>
</div>
<p>Of course, if you have to rewrite most of the methods, instead of just a
few, then you might as well define your own struct that includes the
swig-wrapped object, instead of adding methods to the swig-generated object.</p>

<p>This only works if your wrappers do not need to import other go modules.
There is at present no way to insert import statements in the correct place
in swig-generated go.  If you need to do that, you must put your go code
in a separate file.</p>
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